Field of the Invention
The invention relates to an imaging radar system and, more particularly, to a three-dimensional imaging radar system and method based on flight spectrum.
Description of the Related Art
Three-dimensional imaging radar technique can be widely applied to different fields, such as an anti-collision system of vehicles, a photographing measurement system on freeway, a range-measuring telescope, and machine vision. A three-dimensional imaging radar is an imaging system capable of measuring ranges, and the system includes an emitting part, a receiving part, and an information processing part. At present, the range-measuring principle of the radar includes fight time measurement, phase difference measurement, and triangulation.
The first measurement is the fight time measurement. In this measurement, a pulse light source is used, and a distance to a target object is calculated by measuring the time difference between the time of emitting the light pulse and receiving the reflected light pulse. This measurement can reach the better accuracy, generally reaching the accuracy in a centimeter level in a scope of several kilometers. However, to realize the three-dimensional imaging with a high resolution, points need to be scanned one by one, which is the most commonly used laser imaging radar at present, with a very low imaging speed and a very bad imaging resolution. In another way, a planar array detector with each unit capable of detecting pulses and counting time may be used. For example, an intensified CCD (ICCD) with high-speed modulation may be disposed before the imaging elements, which is applied to the three-dimensional radar without scanning laser. The measurement accuracy of this way is limited to the shape of the light pulse, the imaging resolution is limited to the image intensifier, and the manufacturing cost is high, which may be only used for military and national defense.
Another measurement is the phase difference measurement. In this measurement, a modulation light source is used, and the distance to the target object is acquired according to the phase difference between the reflected light and reference oscillation. Since the phase is limited to 2π, the measuring distance of this measurement is limited to several meters, and the measuring accuracy is not high. At present, there is the radar system via the intensified CCD (ICCD) to realize the planar array phase measurement.
The third measurement is the triangulation. In this measurement, a structure light source is used, and the distance between the object and the light source is calculated according to the light point on the target object and the imaging triangular relation. Although the measuring accuracy of this measurement is high, the applicable measuring distance is shorter. This measurement is usually applied to precious mold manufacture, integrated circuit detection, and SMT circuit board detection. Further, the three-dimensional imaging may be realized by projecting colorful structure light in the two-dimensional space via the light encoded by different colors.
In the above radar range-measuring methods, only the distance information of the single point is obtained. If the target object is to be three-dimensionally imaged, points need to be collected one by one, or the planar array detectors are necessary to collect data in parallel. The present laser radar sensor has certain deficiencies. For example, although the element requirement of the scanned laser radar is lower and the work distance is further, the requirement for the scanning mechanism is higher, the frame rate is lower, and the real-time performance is worse; although the real-time performance of the planar array laser radar is great, the great planar array element is needed for high-resolution imaging, and the cost and research difficulty of the element is high. These laser radars need the light source in the nanosecond level or detectors with quick response.
In recent years, Optics Letters reports that French scientists realize the three-dimensional imaging via the microsecond laser pulse and the high-speed CCD camera based on the intensity integral (OPTICS LETTERS, Vol.32, 3146-3148, 2007). The cost of this measurement is lower than other planar array technique. However, since expensive elements such as lasers are used, the total cost is still higher, and the detecting distance and accuracy is greatly limited.